Convinced that sprawl begets smog, California Central Valley air quality officials are expected today to become the first regulators in the nation to force builders to pay air pollution fees for new development.

Builders would pay less if their new homes, shopping centers and office complexes are designed in ways that limit automobile use — by locating banks and dry cleaners closer to houses, for example, or linking bicycle trails and walking paths to schools and work centers.

The developers could avoid fees if their projects are environmentally friendly enough.

The idea is to prod developers to cut down on traffic in an area where huge growth, and the cars that come with it, have combined with factory farming to create some of America’s dirtiest air.

The proposal is being closely watched by regulators around the country. It pits the building industry, which loathes the idea and fears it may spread, against farm groups — the valley’s other major industry. Builders and some advocates for low-cost housing say the fees will raise prices. Agriculture industry leaders fear that if developers are not required to help clean the region’s air, farmers will bear the entire, costly burden.

The San Joaquin Valley has the highest asthma rates in California and now rivals the Los Angeles basin for the nation’s worst air quality, according to standards set by the Environmental Protection Agency.

Pollution from farms is a major factor in the region’s air problems. Regulators have recently targeted farm emissions — from cow flatulence to wine vapors — to reduce smog.

But overall, motor vehicle exhaust is the valley’s largest source of smog and soot. That pollution is expected to get worse as the region, home to 3.7 million people, doubles in population over the next 40 years.

The table-flat, 400-mile-long Central Valley, of which the San Joaquin Valley is a part, has become a popular real estate destination in the past 20 years as high housing prices in coastal regions have driven thousands inland in search of bigger, cheaper homes. The result, particularly in the San Joaquin Valley, has been a seemingly never-ending expanse of tract homes near towns such as Tracy, often populated by people who commute two hours each way to Bay Area jobs

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